Very recently, while there are strong trends in high density as to optical disks, DVD-ROMs having storage capacities of 4.7 GB have been marketed with respect to CD-ROMs having storage capacities of 0.65 GB, corresponding to commercial-purpose reproducing-only optical disks. DVD-RAMs having storage capacities of 2.6 GB have already been available in actual fields as recordable optical disks having large storage capacities. Within a front half yearly period in HEISEI-era of year 12 (A.D. 2000), DVD-RAMs having larger storage capacities of 4.7 GB will be positively marketed. Such recordable DVDs, needs of various applications are made in such fields as, not only utilizations as storage media designed for computers, but also storage media capable of recording video images without rewinding/fast-feeding operations. At the end of HEISEI-era of year 11, video recorders with employment of optical disks have already been marketed. As to video records with employment of DVD-RAMs, DVD-RAMs having storage capacities of 4.7 GB are expected to be supported. Such video recorders equipped with DVD-RAMs are strongly expected in the market in view of compatibility between CDs and DVD-ROMs. However, the storage capacities of DVD-RAMs are not limited only to 4.7 GB, but may be desirably increased up to 20 GB, by which high-definition moving pictures may be recorded on these DVD-RAMs for 2 hours in connection with such a trend that satellite broadcasting programs will be produced by using digital techniques.
Recording density of an optical disk is substantially limited by a dimension “λ/NA” of a recording/reproducing optical spot (symbol “λ” indicates wavelength of light and symbol “NA” represents numerical aperture of objective lens). As a consequence, in order to increase a storage capacity, a wavelength of light must be shortened, or a numerical aperture must be increased. As to wavelengths, very recently, development as to blue-violet-colored semiconductor lasers having a wavelength of 410 nm has been progressed. Since the wavelength of the laser used in presently-available DVDs having the storage capacities of 4.7 GB is equal to 650 nm, if such blue-violet-colored semiconductor lasers are merely employed, then storage capacities of approximately 12 GB may be in principle realized. The storage capacity of 12 GB is approximately 2.5 times higher than the presently-available storage capacity of 4.7 GB, namely a square of wavelength ratio. However, in order to further increase the storage capacity of 12 GB to 20 GB, the numerical aperture “NA” must be multiplied by 1.3, namely, the NA “0.6” of the presently-available DVD must be increased up to an NA of 0.78.
As the conventional techniques capable of increasing the NA, for example, there is JP-A-11-195229 (first prior art). In this first prior art, the NA is increased up to 0.85 in maximum by employing the two-group/two-sheet of objective lenses. At this time, when the NA is increased, there are such problems that the aberration is increased which is caused by the shift in the optical system, and by the errors contained in the thickness and the inclinations of the disk base plate. To the contrary, in the above-described prior art, in order to reduce the comatic aberration which is produced due to the disk inclination, the thickness of the base plate is made thin (up to 0.1 mm). Also, with respect to the spherical aberration occurred due to the thickness error of the base plate, the thickness of the base plate is detected from the difference between the focal shift signal derived from the surface of the disk and the focal shift signal derived from the recording plane thereof. Then, the interval between the two lenses is changed based upon this detected thickness so as to compensate the spherical aberration.
Furthermore, there is JP-A-2000-057616 (second prior art) as an other prior art. In this second prior art, as previously explained, the control signal used to compensate the spherical aberration is detected by the difference (subtraction) signal between the focal shift signals based upon the astigmatism method, which are detected by separating the inner side and the outer side of the optical spot on the photodetector. Also, at this time, the summation signal between these focal shift signals is used as the focal shift signal.
In the above-explained first prior art, the spherical aberration is detected in such a manner that the thickness of the base plate is detected from the focal shift signal derived from the surface of the disk and the focal shift signal derived from the recording film plane thereof. However, in this case, since the spherical aberration is not directly detected, there are other problems that the errors readily occur due to adverse influences caused by the deviation of refractive indexes of the base plate and the shift of the photodetector, and the control operation can be hardly carried out.
As will be explained later in detail, in the second prior art, there are such problems that the waveforms of the focal shift signals are largely deteriorated which are caused by the spherical aberration itself, and the focal shift range capable of detecting the spherical aberration is narrow. Furthermore, the offset of the focal shift signal caused by the spherical aberration is also large.
The present invention has been made to solve the above-described problems, and therefore, has an object to provide an optical disk apparatus capable of detecting spherical aberration in higher precision and under stable condition, which is caused by deviation of a base plate thickness and a shift in an optical system, and capable of correcting this spherical aberration, and also capable of detecting a focal shift signal having a small offset so as to record/reproduce an optical disk under stable condition.